update SystemWebViewShell to target version API level 28 (Android P) and make sure it remains backward compatible: i.e. ensure cleartext and user certificates are allowed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6d8de33fc1d2308a01dea2d59b7b58c137d63deb

commit 6d8de33fc1d2308a01dea2d59b7b58c137d63deb
Author: Tim Volodine <timvolodine@chromium.org>
Date: Fri Nov 02 21:33:22 2018

[AW SystemWebViewShell] Increase target_sdk version to 28 (P) and enable cleartext traffic

Update the SystemWebViewShell to target Android P (API level 28).
This is a part of the general effort to update apps to API level 28,
see  crbug.com/898190  and b/115499280.

This patch also ensures that cleartextTrafficPermitted is set to
true to ensure backwards compatibility for the SystemWebViewShell
(and to allow cleartext traffic, e.g. using unencrypted http
protocol).

This patch also updates trust-anchors to include system and user
certificates.

BUG= 898190 

Change-Id: I7adb7dbfce1baf3e7b2cf4860709d10fe295217e
Reviewed-on: https://chromium-review.googlesource.com/c/1296601
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Selim Gurun <sgurun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605057}
[modify] https://crrev.com/6d8de33fc1d2308a01dea2d59b7b58c137d63deb/android_webview/tools/system_webview_shell/apk/AndroidManifest.xml
[add] https://crrev.com/6d8de33fc1d2308a01dea2d59b7b58c137d63deb/android_webview/tools/system_webview_shell/apk/res/xml/network_security_config.xml